Synthesis and Nonlinearity of Triene Chromophores Containing the Cyclohexene Ring Structure
- PDF / 704,364 Bytes
- 12 Pages / 414.72 x 648 pts Page_size
- 13 Downloads / 140 Views
ABSTRACT A series of conjugated donor-acceptor trienes in which the central double bond is incorporated into an unsaturated isophorone, verbenone or chromone ring has been synthesized. In each case, the donor group consists of an amine and an aromatic or heterocyclic ring system, and the acceptor is the dicyanomethylidene group. The nonlinear optical properties of each of the compounds has been measured and correlated with its structure. The dipole moments and molecular hyperpolarizabilities of these compounds, like those of other conjugated polyenes, are large enough to be used as the active components of electro-optic polymers. Unlike other donor-acceptor polyenes, however, these compounds exhibit the thermal stability required for such applications. INTRODUCTION A major challenge in the design of nonlinear optical chromophores for electro-optic (EO) devices is simultaneously achieving acceptable thermal stability, transparency, nonlinearity, and processability in one compound. Thermal stability is typically enhanced by incorporating aromatic rings into the delocalized bridge between the electron donating and accepting groups, but it has been shown that such substitution produces excited states that feature quinoidal bonding patterns in those rings and concomitant loss of aromaticity.' The first excited state in such compounds is thus energetically disfavored, decreasing the hyperpolarizability 03.The nonlinearity can be enhanced to some extent by incorporating quinoidal and aromatic rings into the bridge in pairs, so that the net aromatic stabilization energy upon excitation is close to zero, but the thermal stabilities of such compounds will be compromised. This nonlinearity-stability tradeoff can be evaded to some extent by using aromatic substitution upstream of the electron donor group only.2 This substitution pattern is especially effective when combined with cyanomethylene acceptor groups.3 Nevertheless, the largest measured molecular hyperpolarizabilities have been found in long conjugated polyene systems with few or no aromatic rings between the donor and the acceptor.4 t Current address: Lightwave Microsystems Corporation, 2950 Scott Boulevard, Santa Clara, CA 95054
t Current
address: Department of Chemistry, Kent State University, Kent, OH 44242 243
Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
Unfortunately, such compounds decompose far below the 300 °C required for incorporation into real devices' and are sufficiently red-shifted as to fail the transparency requirements. One class of compounds has been found to evade the nonlinearity-transparency tradeoff somewhat by featuring two low-lying excited states that contribute to P in an additive manner,6 but other systems with that desirable quality remain to be found. Accordingly, it remains of great interest to determine whether the thermal stabilities of unsaturated delocalized systems are irretrievably low, or whether structural modifications can increase decomposition temperatures without decreasing hyperpolarizabilities p
Data Loading...
